a) Field of the Invention
The invention is directed to an arrangement for generating extreme ultraviolet (EUV) radiation based on a gas discharge, i.e., radiation of high-energy photons in the wavelength range from 11 to 14 nm (EUV=extreme ultraviolet range).
b) Description of the Related Art
As structures of integrated circuits on chips become increasingly smaller in the future, radiation of increasingly shorter wavelength will be needed in the semiconductor industry to expose these structures. Lithography machines with excimer lasers which attain their shortest wavelength at 157 nm and in which transmission optics or catadioptic systems are employed are currently in use.
Therefore, radiation sources which further increase resolution with even shorter wavelengths for imaging will have to be available in the future (around the year 2007). However, the optical systems must comprise reflection optics at wavelengths below 157 nm because there are no available materials which are transparent for these wavelengths. When using reflection optics, the numerical aperture is limited and the diversity of optical elements is restricted. The lower numerical aperture of the optics results in decreased resolution of the system which can only be compensated by an even shorter wavelength (by about an order of magnitude).
In principle, both laser-induced plasmas and gas discharge plasmas are suited for generating EUV radiation. Laser-induced plasma requires an energy conversion in two stages: from electrical energy to laser radiation energy and from laser energy to EUV radiation energy. This twofold conversion results in reduced conversion efficiency compared to gas discharge.
With respect to gas discharges, different concepts are pursued in plasma focus devices, capillary discharge devices, hollow cathode discharge devices and Z-pinch devices.
Compared to the other concepts, the plasma focus method has the disadvantage of poor spatial stability because of the formation of plasma. In this connection, U.S. Pat. No. 5,763,930 suggests a variant using a noble gas with lithium as working gas. However, this leads to additional contamination of the surroundings, particularly the insulator.
Another competing concept, capillary discharge, has only a short life and, consequently, limited applicability.
The Z-pinch concept exhibits good characteristics compared to other gas discharge concepts and laser-induced plasmas. In a published technical solution according to U.S. Pat. No. 5,504,795, preionization by means of R-F (radio-frequency) discharge is realized in an insulator tube in which the plasma is likewise ignited subsequently. This high-frequency preionization system is directly coupled to the discharge system and is therefore exposed to plasma radiation and to bombardment by charged particles resulting in a shorter life of the insulator tube in particular.